Wasted or stranded natural gas is often vented, flared or shut in due to poor economic conditions. Some natural gas is extracted together with conventional oil—also known as associated gas. Often this gas is in remote locations and it is uneconomical to build a pipeline to collect the gas or collect the gas (and/or waste) via truck. Therefore, this gas may be flared (flared gas) and under circumstances where environment regulations are strict, the production wells are shut in. Transforming the natural gas into useful products—methanol, diesel, gasoline, solvents, or any other hydrocarbon—is an attractive opportunity to reduce CO2 emissions due to flaring and producing an economically viable end-product.
As an example of an end-product, diesel is a petroleum-based fuel derived from conventional reserves, heavy oil bitumen as well as from natural gas at a very large scale. Producing diesel at small scale has been economically unattractive due to the large investment costs required. Commercial units are on the order of 100 000 barrels a day production and smaller units in the range of 5-10 000 barrels a day have been proposed. Reducing the investment and operating costs is critical to commercializing a process at production rates of less than 1000 barrels a day.
Converting natural gas to diesel or other end-products conventionally involves a multi-step process.
In a first step, the oil and gas are separated as they come out of the well and the natural gas is treated to remove impurities including sulphur compounds (H2S, COS, etc.).
In a second step, natural gas and an oxidant are compressed and then the natural gas is converted into synthesis gas (also called syngas, which is mostly a mixture of CO and H2). Many technologies have been proposed for this step.
Most common technologies are based on steam methane reforming (SMR) in which water vapour is fed together with methane over a Ni based catalyst. To use this technology, both sulphur impurities and higher hydrocarbons must be removed as they can poison the catalyst and cause carbon build up. Furthermore, the reaction is highly endothermic such that as much as 20% of the natural gas is required in order to maintain the reactor at about 900° C. The SMR reaction results in a H2:CO ratio of 3 to 4:1, that is an excess of hydrogen as will be discussed below.
Besides SMR, both Auto Thermal Reforming (ATR) and partial oxidation (POX) are used to produce synthesis gas. ATR uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. In this reaction, the methane is partially oxidized. The reaction is exothermic due to the oxidation. When the ATR uses carbon dioxide the H2:CO ratio produced is 1:1; when the ATR uses steam the H2:CO ratio produced is 2.5:1, which again is an excess of hydrogen. The main difference between SMR and ATR is that SMR uses no oxygen. The advantage of ATR is that the H2:CO can be varied. POX is a process in which natural gas or a heavy hydrocarbon fuel (heating oil) is mixed with a limited amount of oxygen in an exothermic process. The general reaction equation is:CnHm+n/2O2→nCO+m/2H2(with catalyst,CPOX).
The last step of one type of end-product is the actual production of diesel through a Fischer-Tropsch (FT) reaction. The Fischer-Tropsch process, converting synthesis gas to diesel, is conducted at low temperature—approximately 220° C. with a cobalt based catalyst or at a slightly higher temperature (300° C.) with an iron based catalyst. The reaction stoichiometry is:nCO+(2n+1)H2→CnH(2n+2)+nH2Owhere n varies from 2 to 40. The produced gases condense resulting two liquid phases. The bottom aqueous phase is predominantly water and the less dense top-organic phase is comprised of C4+ hydrocarbons.
This reaction optimally requires a ratio of 2.1-2.3:1 (H2:CO). Hydrogen beyond this ratio will react with CO in the diesel step (FT) to form methane. Therefore, prior to the FT step, excess hydrogen must be removed either through membrane technology, pressure swing absorption or through the Water Gas Shift Reaction.CO+H2O→CO2+H2(Water Gas Shift Reaction)
Current technologies for producing diesel from natural gas are capital intensive, require multiple steps, and reagents to achieve yields no better than 50%. Together with the extra vessels, piping, valves, flow meters, fittings, are also required. Furthermore, during start-up, provision must be made to heat the reactors up from ambient conditions to reaction conditions. This is accomplished with start-up burners (typically methane). The start-burner also requires investment including additional piping, valves, flowmeters and associated safety equipment and control measures.